There are two types of papers. The first with many authors and 2nd one with single author. Why people prefer single author publication than that of many authors?
Well, generally, there is no choice. Single authorship tends to be nearer the norm in the humanities and social sciences. Joint authorship is more usually the case in the natural sciences. Your choice is which do you want? For single authorship, you need a good wide command of the subject. For joint, you need others you can trust.
To the reader it seems of secondary importance whether an essay had been presented as the product of one or more authors. If you are interested in the topic and the content and data satisfies your needs, you could hardly ignore a report even if you had a preference for single authors. There are, however, some issues one might want to address. First off: what types are there?
Written by single author
Written by single author on behalf of a research group
Presented as written by a number of authors, but actually written by single author
Single author adds non-contributors as authors due to them either demanding it or him trying to profit from their status
The multiple-author report might just as well have been written by one author, based on research that has been conducted by the whole lot of them, but it might just as well have been written by one author, without any input or contribution, no matter whether by providing research-data or text-bits. There are people of a higher standing who are able to force their way into authorship without providing anything and there are people who feel they have to accept it. Misuse of power we are talking about here.
In some cases a report might actually have been written by the said number of authors. Even though this might work with two authors, if they decided on specific chapters for each of them and wrap up their ideas in a collectively written introduction and conclusion, many authors who actually write a text together might end up with an unreadable product. Working like that is no easy thing to do and one might find himself in heated debates about lenght of sentences and wording. Writing styles are altogether quite different and one has to compromise a lot. For the author himself being responsible for the whole text is the preferred situation to be in.
For the reader, I believe, one author has a lot of advantages. You know whom to quote. Whom do you quote if there were given five authors? You do not know who expressed a thought. To cut things short: I do prefer single authors. I do prefer being a single author, or one of two at the most and if asked whether somebody should write an essay by himself or as a team I would strongly suggest to either write it himself or at least let them have single chapters that can be identified as having been written by single authors, so that each of them has their designated areas of responsibility. If there was, however, a report I was strongly interested in and it happened to be by a bunch of authors and there was no way around it, I would have to read it because of the importance of the topic.
Having said all that, adding names of authors who did not contribute in any way for whatever reason, is completely out of the question and one should never do it. It is practically accepting that others steal from you. You even hand it to them. It is misleading to the readers too because they believe a certain topic had been dealt with by an author who actually did not contribute. In agreeing to add non-contributors you do not only violate your own rights but act against your readers. You are willingly misleading them.
And there we have it. Once a reply turned sub-chapter. Just bear with it.
You are right that adding names without any contribution is not the right way. But this trend is very common almost everywhere to be in race for impact factor. Although its not a professional way to deal with science. Researchers talk very much on ethics in Science, but when it comes in the way of impact factor than everything get vanish.